1. Field of the Invention. The subject invention relates to connectors for connecting integrated circuit packages to printed circuit boards, and more specifically, a connector for establishing solderless connections between the leads of an integrated circuit package and a printed circuit board.
2. Description of the Prior Art. Integrated circuits are typically housed within a package which is designed to protect the integrated circuit from damage, provide adequate heat dissipation during operation, and provide electrical connection between the integrated circuit and the leads of a printed circuit board. Several conventional packages are in the prior art including land grid array (LGA), pin grid array (PGA), ball grid array (BGA), and column grid array (CGA).
In integrated circuit (IC) packages, terminal lands are arranged on one major face of the package in a pattern corresponding with mounting pads, or leads, on the surface of a circuit board or the like. The device package is mounted on the circuit board by soldering the terminal lands to the mounting pads. Packages having a pattern of lands distributed over a major portion of one face thereof are called land grid array (LGA) packages. Similarly, packages having small solder bumps arranged in a pattern on one face for forming interconnections with external circuitry are usually referred to as ball grid array (BGA) packages.
In many applications, the soldering of the leads of the IC package to the printed circuit board is undesirable. For example, it is impossible to visually locate a short or ground between the IC package and printed circuit board. Usually, an expensive X-ray technique is required to inspect the connections since the leads are hidden under the package. Further, the increasing number of leads being provided by IC packages makes the soldering of the packages to printed circuit boards more difficult.
Accordingly, in the prior art, an improved connector has been developed which is designed to eliminate the need for the soldering the leads of an IC package to a printed circuit board. One example of a device which satisfied this criteria is the xe2x80x9cfuzz ballxe2x80x9d socket. The xe2x80x9cfuzz ballxe2x80x9d socket comprises a non-conductive substrate formed with a plurality of through holes which each house a contact element. The contact elements are formed by forcing a predetermined length of gold plated wire into a through hole such that the wire will bend haphazardly into a jumbled contact that extends through the through hole and resembles a piece of steel wool. To mount an IC package to a printed circuit board, the xe2x80x9cfuzz ballxe2x80x9d socket is tightly secured to a printed circuit board and, in turn, the package is tightly secured to the xe2x80x9cfuzz ballxe2x80x9d socket. It can be appreciated, sufficient pressure must be applied to both the xe2x80x9cfuzz ballxe2x80x9d socket and the package, respectively, to maintain electrical connections between the lands of the package and the printed circuit board via the xe2x80x9cfuzz ballxe2x80x9d socket.
As the number of lands and corresponding xe2x80x9cfuzz ballxe2x80x9d contacts are increased, the pitch between contacts is correspondingly decreased and manufacturing problems increased. The placement of individual wires into evermore tightly packed through holes requires tremendous technological developments. Furthermore, xe2x80x9cfuzz ballxe2x80x9d sockets are relatively expensive due to costly manufacturing including the placement of individual wires into the through holes to form the various xe2x80x9cfuzz ballxe2x80x9d contacts. Additionally, the great force required to push the ball leads of a BGA package into contact with the xe2x80x9cfuzz ballxe2x80x9d socket creates wear on the BGA ball leads and increases the likelihood of distorting the ball leads.
In addition to the need for a socket which requires little or no force during insertion and a large number of contacts as the pitch of these contacts decrease, a connector is desired which employs resilient contacts that work reliably over repeated cycling and extreme temperature fluctuations as is encountered during testing and burn-in. Prior art connectors employing xe2x80x9cYxe2x80x9d, xe2x80x9cpinchxe2x80x9d and xe2x80x9cforkxe2x80x9d contacts satisfy the resiliency requirement necessary for burn-in but will not accommodate the tight tolerances and miniscule features of packages with a 0.5 mm-pitch, such as in chip scale packages (CSP). Alternatively, conductive elastomers have failed since the conductive materials yield after a limited number of cycles and a variation in the planarity of the gird array among different packages results in intermittencies. Furthermore, the elastomers tend to yield when exposed to high temperatures.
To overcome the shortcomings of the prior art, it is an object of the subject invention to provide a connector for solderless connection between an IC package and a printed circuit board.
It is another object of the subject invention to provide a connector which reduces the large amount of pressure required to mount an IC package into a socket.
It is a further object of the subject invention to provide a connector having a unique resilient electrical contact capable of achieving an electrical connection between the contact of a circuit board and a lead of an IC package, regardless of whether the IC package is a BGA or LGA package.
It is still a further object of the subject invention to provide a connector having a unique resilient electrical contact which will not deform a ball lead of a ball gird array (BGA) package.
It is also an object of the subject invention to provide a connector which when mounted to a printed circuit board with an IC package secured thereto, the spacing separating the IC package from the printed circuit board is virtually equal to the thickness of the non-conductive substrate of the connector.
To meet the above-stated objects, a connector assembly is provided for solderlessly connecting an IC package to a printed circuit board. The connector assembly of the subject invention includes a non-conductive substrate formed with a plurality of through holes, each through hole corresponding to a land of the integrated circuit package. In one embodiment of the invention, a generally cylindrical resilient electric contact is disposed within each of the through holes to form an electrical connection between the corresponding land and the lead of a printed circuit board. Additionally, as a second embodiment, a resilient electrical contact formed from a spring to resemble two cones joined at their bases is provided within each of the through holes.
With respect to the first embodiment, the generally cylindrical resilient electrical contacts are formed from a single unitary conductor. The conductor is coiled in a helical fashion to form a contact which resembles a spring. Each spring-like contact is formed to begin with an annular rim defining a contact point. The conductor is then coiled in such a manner to form diametrically equal rings which are spaced equidistant apart. The spring-like contact is then concluded with a similar annular rim as to the beginning contact point. An intermediate portion equidistant from beginning and concluding rims is formed so that a small portion of coiled turns have a diameter which is larger than the other diameters defined by the annular rim. The enlarged intermediate portion of the spring-like contact engages the inner surface of the through holes in an interference fit. The beginning annular rim defines a contact point for receiving and electrically engaging a ball lead or land of a grid array package, while the concluding rim is for engagement with an underlying printed circuit board or semi-conductor device. The spring like contact may be formed from any known resilient conductive material, such as heat-treated beryllium copper. Preferably, the contact is coated with gold, nickel or the like to assure high flexibility, resiliency and electrical conductivity.
The spring-like contacts are mounted within a substrate consisting of two layers each of which includes through holes having at one end thereof an inverted truncated enlarged portion. When the layers are stacked and the contacts mounted therein, the enlarged intermediate portion of the spring-like contact is captured within the inverted truncated opening. In effect, the enlarged intermediate portion of each contact is captured and non-extensible, whereas the opposite ends thereof are resilient and compressible when the connector is in the operative position.
In an unbiased state, the annular rims of the spring-like contact extend slightly beyond the respective outer surfaces of the non-conductive substrate and the coiled turns are spaced equally apart. During use, the IC package and printed circuit board are respectively placed in tight face-to-face engagement with the non-conductive substrate, thereby, compressing the electrical contacts into the non-conductive substrate. The compressed electrical contacts form electrical connections between the IC package and the printed circuit board. Spring forces reactive to the compression of the electrical contacts, maintain the annular rims of the electrical contacts in tight engagement with the corresponding lands and leads. Also, due to the compression of the electrical contacts, in use, the spacing between the IC package and the printed circuit board is virtually equal to the thickness of the non-conductive substrate. Consequently, the connector and the IC package combination can be advantageously assembled and mounted to substantially encompass the actual sum of the dimensions of the IC package and the connector. With space and volume within a computer being at a premium, dimensional increases added to an assembled, mounted component resulting from the mounting procedure are undesirable.
Due to the characteristics of the contact being a spring, it can be appreciated that, when the connector is utilized, the contact only moves in the vertical direction. As opposed to prior art contacts of the xe2x80x9cpinchxe2x80x9d type where the contact arms move horizontally to grab a lead, the contact of the subject invention will not be displaced in any horizontal direction. Since the through hole does not need to be oversized for the flexing of contact arms, the through hole of the new and improved connector needs to be only slighter larger than the diameter of the turns of the coils of the contact. Advantageously, this will allow the through holes to be smaller than prior art through holes and will allow the pitch of the through holes to decrease being optimally suited for chip scale packages (CSP).
In the mating of a BGA package, the annular rim contact point of the spring-like contact receives the conductive balls leads without undue pressure, and due to the compatible shapes of the annular rim and spherical ball leads, the likelihood of distorting the ball lead is decreased. Also, due to the flat plane the annular rim is defined in, the spring-like contact is desirable for use with land grid array packages, pad grid array packages and the like. In all cases, the spring-like contact allows electrical connection between the contacts of a circuit board and the leads of an IC package without soldering.
In the second embodiment, the conductor is coiled in a helical fashion similar to the contact described above but each turn is formed with a variable diameter. The conductor begins with an annular rim defining a contact point and is then coiled with diametrically increasing rings until an intermediate point of a predetermined length is reached. The rings will then diametrically decrease until a ring is formed with substantially the same diameter as the beginning annular rim. The completed contact will resemble two mirror image cones connected at their bases. Similarly, to the connector of the first embodiment, the substrate retaining the contacts will be formed with through holes to capture the double-end cone-like contacts. The operation and functionality of this embodiment is similar to that described above with respect to the first embodiment.
These and other features of the invention will be better understood through a study of the following detailed description of the invention and the accompanying drawings.